A variety of equipment are known suitable for grinding, i.e. pulverization of material like coal, lignite, cement etc. For the pulverization of coal used in boilers of thermal power plants, a limited number of such equipment are commonly used. For the purpose of discussion that follows, a coal fired system mainly consists of a coal feeder, equipment for pulverizing coal, a distribution system for distributing the coal after the pulverization thereof, a furnace in which the coal is to be burned, and the requisite controls for effecting the proper operation of the coal fired generation system. The equipment for pulverization of coal is of particular interest here. The coal pulverizing equipment are known to exist in prior art for more than sixty years. Many improvements in the construction and/or mode of operation of these equipment have been made during this period.
The coal pulverizing equipment is chosen based on the advantages it offers n terms of reliability, low power consumption, minimum maintenance, a wide range of capacity, low rate of wear of the replaceable grinding elements used with uniform throughput of the required range of pulverized coal fines during their useful life. In addition, it should have an integrated lubrication system, convenient adjustment and control of coal flow and fineness, ability to handle high temperature air that is required for moisture coal and quiet operation.
Bowl mill is one such equipment which is commonly used in fossil fired boilers of thermal power plant, which has the above mentioned features to different desirable degrees. The name is obviously derived from the fact that the pulverization takes place on a grinding surface which resembles a bowl. Reference can be made to U.S. Pat. No. 3,465,971 (1969) and/or U.S. Pat. No. 4,002,299 (1977) which give the nature of construction and the mode of operation of a prior art form of bowl mill that is suitable for use in a coal fired power generation system for the pulverization of the coal.
Coal fired boilers used in thermal power plants require majority of the pulverised coal in a fineness range around 75 microns size for efficient combustion. Various types of pulverisers are employed in Thermal Power Stations. Raymond Bowl mill is one commonly used pulveriser for this purpose. The main components of a bowl mill consists of a cylindrical body, usually known as separator body, within which a conical bowl like grinding table is mounted for rotation, three equally spaced grinding rollers in the shape of inverted frustum of a cone are mounted so as to interact with the rotating bowl to grind the coal lumps fed on to the centre of the bowl into fine powder like form. Three rollers are mounted on to inclined stationary axis journal shafts by berings so that the roll surface is aligned with the bowl surface and are able to rotate about the axis by the friction force derived from the rotating bowl. Preselected force can be applied between the bowl and the roll surface by means of spring or hydraulic arrangement to assist grinding. The ground particles are thrown out of the bowl by centrifugal force derived from rotation of the bowl.
The hot primary air stream enters the mill from bottom of the mill known as air mill or air inlet housing, usually in horizontal direction through a single entry of rectangular cross section, such that entry air velocity is tangential, in the horizontal plane, to the annular space of the air mill. The hot primary air moves into the annular space which is the low velocity air plenum in the air mill, and is then accelerated and oriented by a series of stationary or rotating passages in a ring that surrounds that grinding zone. These arrangements for directing the primary air through narrow annular area between the bowl rim and the housing are known as stationary or rotating vane wheel depending on their kinematic disposition. There are many relevant patents on the subject of vane wheel in which it is claimed that the vane wheel arrangement directs the air stream upward through the annular passage between the bowl rim and the housing in an efficient manner. U.S. Pat. No. 4,264,041 (year 1981) titled “Low Pressure Drop Pulverizer Throat”, describes a new and improved design of pulverizer air throat for low pressure drop, reduced erosion and improved flow distribution. U.S. Pat. No. 4,523,721 (year 1985) titled “Bowl Mill with Primary Classifier Assembly”, describes a rotating vane wheel arrangement for change in the direction of flow, counter clockwise to the direction of rotation of the grinding table which is helpful for primary classification of coarser coal particles. U.S. Pat. No. 4,687,145 (year 1987) titled “Roll and Race Pulverizer with rotating throat” summarises the invention as a roll and race pulverizer in which the throat vanes are mounted for rotation in the direction of rotation of the grinding ring. The rotating vanes are claimed to be spreading the ground coal spilling on to the throat area. There are other patents like U.S. Pat. No. 518,404 (year 1983), U.S. Pat. No. 5,020,734 (year 1991), U.S. Pat. No. 2,378,681 (year 1945), U.S. Pat. No. 2,545,254 (year 1951), on the subject. In the U.S. Pat. No. 5,263,655 (year 1993) titled Coal Pulverizer an improved annular passage arrangement for pulverizer has been advocated to reduce the pressure drop across the vane wheel and also thereby to provide means of directing passage outlet airflow away from the grinding table. It is also claimed to be improving the distribution of air velocity across the annular passage.
At the outlet of the passage near the bowl rim, the pulverized coal particles are entrained by high speed air flow. The heavier coal, stone or quartz particles fall down through the annular space between the bowl and the body on to the mill base as rejects.
The velocity of air is reduced above the bowl rim area in the main pulverizer housing causing the larger particles to be returned directly to the grinding zone for further crushing, while the smaller particles are carried up through the classifier for final sizing. The moisture present in the ground coal particles are also dried in the process of association with hot air. The ground coal particles carried to classifier, get classified and the lighter particles of desired fineness go to the different elevation of the boiler through four outlet ports and the heavier Particles fall back on to bowl for further regrinding continuing the cycle of pulverization.
U.S. Pat. No. 3,465,971 issued in 1969 titled “Deflector Arrangement for Use in a Grinding Mill”, disclosed construction of a deflector for directing the air borne pulverized material leaving the grinding rind downwardly and inwardly back for better primary classification. In this patent the entry of primary air in the air mill is through a single inlet. This type of bowl mills known as Raymond Bowl Mill, have single entry of primary air.
In actual operation of this type of mills, the drawbacks encountered are dissimilar wear on the three rolls., high rate of rejects and non uniform output from the outlet ports. These drawbacks are suspected to be due to non uniform air flow inside the mill and also inadequate air velocity in certain areas inside the mill. It is also found that the clinker formation in the airmill are due to formation of eddies and vortices in the air mill.
Analytical studies using computational fluid flow software package reveals that the distribution of airflow in the air mill section is not uniform around the periphery. It is seen in such a study that with single entry of primary air into the mill only about 60 percent of the total cross sectional area has high velocity. Further away from the inlet, the velocity of air loses its magnitude to very low value compared to the velocity at the entry region. This essentially means that in abort 40 percent of peripheral area of the bowl, the coal particles will not be lifted with desired velocity and will result in falling back on the bowl for regrinding. Another effect could be heavy rates of rejects containing coal lumps and other particles through that part of the throat area where the vertical velocity component of air is less. Such a study also shows the presence of considerable eddies and vortices, specially in the last quadrant of the air mill section from the entry.
The purpose of introducing rotating vane wheel in some of the patents described in earlier paragraphs appears to be to remove the deficiency of the non-uniform distribution of air velocity around the bowl rim coming from the air mill. This is attempted by spreading the incoming air and also the falling pulverized coal, which is possible only to a limited extent by the rotating vanes attached to the bowl. This effect can only be very localized as the fanning action can not bring about a high degree of uniformity where the peripheral velocity of the bowl is much less compared to the average vertical velocity of the primary air near the bowl rim.
Considerable improvement in primary classification is possible by uniform flow distribution reducing regrinding. Similarly minimization, of eddies and vortices will lead to cutting down energy losses and also avoid accumulation of inflammable rejects which lead to clinker formation.
It is, therefore, an object of the present invention to achieve much better uniformity in airflow around the air mill section of a bowl mill of a coal pulverizer, for having uniform output from the outlet ports and for avoiding dissimilar wear on the rolls.
Another object of the present invention is to minimize the eddies and vortices formed by the non uniformity of flow for avoiding clinker formation.
Yet another object of the present invention is to reduce the amount of undesirable rejects from the mill.
A still further object of the present invention to reduce the overall energy consumption and make the mill operation more efficient.